The present invention relates to an impregnated ceramic core suitable for use in investment casting and a method of impregnating the fired porous ceramic core to strengthen the same.
Ceramic cores suitable for use in investment casting may be made by an injection-molding process. A mixture of ceramic particles and a binder material (such as wax) is prepared and formed into the desired ceramic object (here, the porous core). The formed core is then fired, sometimes in one firing operation, but commonly in two firing operations, a low-temperature firing (at about 250.degree. F.) and a high-temperature firing (at about 1600.degree. F. or higher). The low-temperature firing removes from the core a major portion of the binder material which, depending upon its composition, either sublimes or melts and flows away. The purpose of the low-temperature firing is to remove the bulk of the binder material so that there is less dimensional change during the high-temperature firing. The high-temperature firing is performed at a temperature sufficiently high to sinter and therefore harden the ceramic particles, removing any remnants of the binder material which were not removed during the low-temperature firing. After the high-temperature firing, the porous ceramic core is cooled, inspected and optionally cleaned. It is then ready to be forwarded to the investment caster at a foundry for use in investment casting.
However, the ceramic cores thus produced are intrinsically brittle and of relatively high porosity (20-45%); accordingly, they exhibit a low modulus of rupture (M.O.R.) or flexural strength. For investment casting, both the flexural strength and the degree of interconnective porosity must be carefully engineered. For example, during the wax-injection stage of the investment casting process, the core must maintain structural integrity. However, the combination of high porosity, complex shape (e.g., as required for airfoils) and possibly extremely thin cross sections (on the order of a few mils) tend to render the cores susceptible to distortion and/or fracture. On the other hand, if the strength of the core is too great during the metal-solidification stage, a defect commonly known as "hot tearing" will result.
Thus, in order to impart structural integrity to the core for the wax-injection step, while still maintaining the desired engineering properties during the casting (metal-solidification) step, the core is often impregnated with a thermoset resin/binder and then heat cured (e.g., at 250.degree.-400.degree. F.) to impart strength to the porous ceramic core during handling, shipping and, ultimately, the initial stages of investment casting (i.e., the wax-injection step). Since the core manufacturer and the investment caster are typically found in different locations, en important function of the resin/binder system is to afford sufficient strength to the porous ceramic core to enable it to be handled and shipped from the core manufacturer to the investment caster at the foundry. The impregnation agent is typically a wax, such as caranuaba, or a phenolic resin to impart strength to the core. After the wax-injection step, the resin/binder is typically pyrolyzed in order to reestablish the core's intrinsic physical properties and, in particular, the strength and porosity required for the subsequent processing stages (that is, the casting and chemical leaching steps).
In the investment-casting process, the ceramic core is frequently required to maintain close dimensional tolerances which, depending on the particular core geometry and the intended application, may be on the order of a few mils (e.g., 0.01-0.02 inches). Conventional resin/binder-treated cores exhibit a high degree of core distortion (e.g., 10-20 mils equivalent to 0.2-0.5 millimeters) which frequently exceeds the dimensional tolerances and thus requires rejection of the core. While the exact cause of this distortion is not known with great particularity, it is believed to be related to the curing mechanism used to cure the resin/binder system and/or the interactions between the core material and the resin/binder material. However, regardless of the exact mechanism causing such distortion, clearly this distortion must be held to a minimum.
The conventional resin/binder systems have not proven to be entirely satisfactory in use. Typically, the resin/binder system utilized presents disadvantages in terms of the low level of additional strength afforded to the core, the requirement for curing of the resin/binder after impregnation with its resultant distortion of the core, various industrial safety and environmental concerns (e.g., toxicity) and cost.
Accordingly, it is an object of the present invention to provide a method of greatly enhancing the strength of fired ceramic cores while at the same time maintaining stringent dimensional tolerances.
Another object is to provide such a method which does not require a curing step.
A further object is to provide such a method which eliminates various industrial safety and environmental concerns and is characterized by a low cost.